Add type definitions for :buffer_in and :buffer_out

Not sure if specs should be added here. `:buffer_in` and `:buffer_out` are just another names of the same type (pointer), i.e. go into the same class of equivalence.

Rubinius actually has specs which use `:buffer_in` and `:buffer_out`, but those specs also rely on `FFI::Buffer`, which is not implemented yet.